The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a system and method for improved water and fat separation using a set of low-resolution MR images to correct phase errors with overall reduced scan time and processing time and enhanced reliability.
Fat-suppression in MR imaging is useful for improved image contrast in regions containing adipose tissues. Areas of applications of the fat-suppression include, but are not limited to, imaging of cartilage, optical nerves, breast, liver and adrenal masses. In addition, quantitation of the relative contents of water and fat, which requires both water and fat images, can be valuable for diagnosing bone marrow diseases and for characterizing atherosclerotic plagues.
In a clinical environment, fat-suppression is currently performed mainly with two techniques, namely Chemical Saturation (ChemSat) and Short TI Inversion Recovery (STIR). However, both require an additional RF pulse applied before the regular imaging sequence and each has fundamental limitations. In ChemSat, fat-suppression is achieved by applying an excitation pulse with a narrow frequency-bandwidth, followed by spoiling gradients. As such, this method is intrinsically sensitive to magnetic field B.sub.0 inhomogeneity. In STIR, the additional RF pulse applied is a 180.degree. inversion pulse with the inversion period TI set to the fat nulling time (TI=T.sub.1 In2, where T.sub.1 is the longitudinal relaxation time of fat). Although STIR is less sensitive to magnetic field B.sub.0 inhomogeneity, it alters the normal image contrast, lowers the overall image signal-to-noise ration (SNR), and sometimes becomes useless because it also suppresses the signals from water with a similarly short longitudinal relaxation time.
Another approach for fat-suppression is commonly referred to as the Dixon technique and involves obtaining a first image, for which water and fat magnetization vectors are parallel, and a second image, for which the two vectors are anti-parallel. Summation of these two images in the complex form yields a water-only image, and subtraction of the two images yields a fat-only image. Unfortunately, such simple treatment also breaks down in the presence of field inhomogeneities. The fundamental challenge for the Dixon-type technique lies in correcting the various phase errors of the complex images. It was later recognized that the field inhomogeneity-induced errors can, at least in principle, be corrected through modified data acquisitions and image reconstruction algorithms. Because of the promise of this technique, several variations to the original Dixon technique were developed. Typically, more data acquisition and more sophisticated reconstruction algorithms were used before the image summation and subtraction. Despite these efforts, the so-called Dixon techniques have not acquired widespread commercial acceptance. The major disadvantages of these techniques are that they generally require long imaging time because of multiple data acquisitions, and the algorithms used for correcting the phase errors are too time-consuming for on-line implementation. Further, these algorithms often lack the robustness for general clinical use and sometimes require manual intervention.
It would therefore be desirable to have a system and method for water and fat separation in an MRI that can be accomplished in a clinical setting with shorter overall scan times and enhanced reliability.